1. Field of the Invention
This invention relates to methods for producing optical fiber preform used as starting material for producing, by wire drawing, fiber-optic bundles for forming optical transmission lines, and more particularly to a method for producing an optical fiber preform consisting in causing glass material soot (hereinafter referred to as soot) formed by a CVD (Chemical Vapour Deposition) method to be deposited on the target to form a soot rod, and heating the soot rod to render same transparent.
2. Description of the Prior Art
An outside vapor oxidation deposition method and an axial deposition method are known as methods for producing optical fiber preform by causing soot to be deposited on the target to form a soot rod (a rod-shaped compact body of pulverized glass) and heating the soot rod to render same transparent.
The outside vapor oxidation deposition method will first be described by referring to FIG. 1. An oxyhydrogen flame 3 containing source gas which forms glass upon reaction is blown through a burner 4 against a rotating seed glass bar 2, the burner 4 being movable. The soot in the flame 3 is deposited on the rotating seed glass bar 2 to form a soot rod 1. The soot rod 1 is rendered transparent by subjecting same to calcination and then heating same at elevated temperature after withdrawing the seed glass bar 2. 5 designates a burner movement guide.
The axial deposition method will be described by referring to FIGS. 2 and 3. In the method shown in FIG. 2 which is described in Japanese Pat. application Laid-open No. 107313/76, soot is deposited on a rotating target 15 axially thereof. More specifically, soot is directed axially of the target 15 as a flame 12 is blown from a burner 11, so that the soot will be deposited on the target 15 which moves downwardly while rotating. The target 15 is enclosed in a glass tube 14 which is in intimate contact with a soot rod 13 formed on the target 15 by the deposited soot, so as to control the diameter of the soot rod 13. After the glass tube 14 is filled with the soot rod, the rod is heated to render the soot rod transparent and unitary with the glass tube 14, so as to produce an optical fiber preform.
FIG. 3 shows a method wherein a flame 25 is blown from a burner 26 axially of a target 21 which moves axially while rotating so as to cause soot to be formed in the flame 25 and deposited axially on the target 21 to form a soot rod 24, and the soot rod 24 is continuously heated in a furnace 23 and rendered transparent, so as to produce an optical fiber preform 22. The method shown in FIG. 3 is similar to the method shown in FIG. 2 in that the soot is deposited axially of the target which moves axially while rotating, but the former is distinct essentially from the latter in that the soot rod alone constitutes the optical fiber preform instead of the glass tube and the soot rod together constituting the optical fiber preform and that the soot rod is rendered transparent by continuously heating it following its production.
Some disadvantages are associated with the methods of the prior art described hereinabove. These disadvantages are as follows.
The outside vapor oxidation deposition method shown in FIG. 1 requires an additional step of withdrawing the seed glass bar, thereby making it impossible to render the soot rod transparent in a continuous operation of forming a soot rod and giving transparency to the soot rod.
In the axial deposition method shown in FIG. 2, a difficulty is encountered in many operations in causing soot deposition to be effected in such a manner that a soot rod is formed that has a diameter which is equal to the inner diameter of the glass tube. Meanwhile the axial deposition method shown in FIG. 3 has the following disadvantages:
(1) Difficulties are experienced in causing dense soot deposition to take place in the outer marginal portion of a soot rod, and consequently the outer marginal portion of the soot rod gets porous, with a result that the soot rod has a large diameter than would be the case if dense soot deposition could be effected in the outer marginal portion of the soot rod.
(2) The distribution of refractory index of an optical fiber preform obtained by heating and rendering transparent a soot rod intended for producing fiber-optic bundles which show a graded index type refractory index is, as indicated by a curve 31 in FIG. 4, deflected at the outer marginal portion of the soot rod from the parabolic profile indicated by a curve 32 which is usually desired.
(3) Control of the diameter of the soot rod formed is not effected satisfactorily.
In this connection, it is added that the aforesaid deflection of the distribution of the refractive index of the optical fiber preform from the parabolic profile in the marginal portion of the soot rod as shown in FIG. 4 could be obviated if the diameter of the soot rod could be reduced without altering the rate of axial movement of the target and the condition of the burner.
Japanese Pat. application Laid-open Nos. 71316/76, 107313/76 and 143037/77 are cited as the prior art having the greatest relevance to the present invention.